1. Introduction
U.S. Pat. No. 6,248,838, “Chain entanglement crosslinked proppants and related uses”; the background section of U.S. patent application Ser. No. 11/323,031 entitled “Thermoset nanocomposite particles, processing for their production, and their use in oil and natural gas drilling applications”; and the background section of U.S. patent application Ser. No. 11/451,697 entitled “Thermoset particles with enhanced crosslinking, processing for their production, and their use in oil and natural gas drilling applications”, provide background information related to the present invention and are fully incorporated herein by reference. The background discussion below is intended to supplement the background discussions in these three prior filings, and focuses entirely on impact modification.
Impact modification has only been given limited and cursory consideration in prior art on fracture stimulation. U.S. Patent Application No. 20040043906 cited impact modifiers among the types of additives that can be incorporated into polymeric proppants in order to control their mechanical properties. U.S. Patent Application No. 20060078682 disclosed particles for use as proppants, where the particles comprise a substrate comprising an inorganic material, and an organic coating (disposed upon the substrate) which may optionally contain impact modifiers intended mainly to impart elastic properties to the organic coating. U.S. Pat. Nos. 5,597,784 and 6,372,678, and U.S. Patent Application Nos. 20050194141 and 20070036977, disclosed fracture stimulation technologies utilizing coated proppants comprising more than one coating layer, where a “reinforcing agent” may be interspersed at the boundary between different layers of the coating, and impact modifiers may serve as reinforcing agents.
Applicant has, however, found no prior art in the patent literature, and no publications in the general scientific literature, that disclose a method for the fracture stimulation of a subterranean formation having a wellbore by using impact-modified ultralightweight thermoset polymer nanocomposite particles as proppants. The discussion below is hence intended to be mainly of a pedagogical nature, by providing background information that will help those in the field understand the invention better by familiarizing them with key information related to impact modification. Since the preferred embodiments of the invention involve the use of thermoset nanocomposite particles having styrenic polymer matrices, this description of information related to impact modification will be done in the context of the impact modification of styrenic polymers.
2. Types of Impact-Modified Styrenic Polymers
High-impact polystyrene (HIPS) is the most commonly used impact-modified styrenic polymer. Copolymers of styrene with other suitable vinylic monomers (such as other styrenic monomers, acrylic monomers, nitrile monomers, monomers containing ion exchange capable functional groups, etc.) have also been toughened. Crosslinked versions of many of these styrenic polymers, with divinylbenzene being the most commonly used crosslinking agent, have also been toughened. Macroporous styrenic beads of various compositions (sometimes crosslinked with divinylbenzene) have also been toughened. See Conway et al. (1997) for the toughening of porous aminated crosslinked poly(vinylbenzyl chloride-divinylbenzene) beads. See Coelho et al. (2000) and World Patent No. WO9607675 for the toughening of copolymers that have been crosslinked with divinylbenzene. U.S. Pat. No. 5,847,054 teaches a method for the preparation of crosslinked styrenic polymer particles containing an impact modifier, for use as additives intended to increase the “dullness” (reduce the glossiness, make more matte) and/or enhance the impact strengths of thermoplastic and thermoset polymers when incorporated into them. U.S. Application No. 20050154083 teaches styrenic particle compositions (possibly crosslinked to a slight extent by using a small amount of a comonomer such as divinylbenzene, and possibly also containing an impact modifier) encapsulating high aspect ratio particles; and intended to be used as pigments which, when incorporated into any of a very wide variety of matrix polymers, will impart attractive optical properties to those matrix polymers.
Syndiotactic polystyrene (a highly crystalline form of polystyrene that has especially high stiffness and heat resistance, but that is even more brittle than ordinary general-purpose polystyrene) has also been toughened (U.S. Pat. Nos. 5,352,727 and 5,436,397) by incorporating impact modifiers.
Styrenic polymers containing fillers of a wide variety of types (ranging from nanoparticles to macroscopic fibers) have also been toughened. The fillers in these polymers provide functions such as reinforcement, densification and/or magnetism (as in World Patent No. WO9607675 which teaches a method for producing toughened crosslinked copolymer beads that may contain solid magnetic particles). There is some evidence that improved dispersion of solid particulate fillers can be obtained if a dispersing agent that reacts to form covalent bonds with (and thus becomes grafted onto) the matrix polymer is used.
Thermoplastic polymer blends with improved impact resistance have been obtained by mixing toughened styrenic polymers with other thermoplastic polymers of interest by techniques such as melt blending.
Toughened styrenic polymers have also been incorporated as additives in other polymers to impart some special properties to the host polymer.
While impact-modified styrenic polymers have been used in many industrial applications, applicant does not believe that these applications include the use of impact-modified styrenic polymers as proppants in the fracture stimulation of a subterranean formation having a wellbore. Nor is applicant aware of any patented or reported methods for the fracture stimulation of a subterranean formation having a wellbore by using impact-modified ultralightweight thermoset polymer nanocomposite particles based on any other type of polymeric matrix material as proppants.
3. Improvements Resulting from Impact Modification
Rubber modification is the most common method for the impact modification of styrenic polymers. It has provided up to several times higher notched Izod impact strength and much higher ductility (as quantified by the ultimate tensile elongation); without large losses in the stiffness (elastic moduli), strength, gloss, or heat distortion temperature. Increasing effectiveness of impact modification in improving the mechanical properties has been shown to correlate with increasing energy dissipation by the deformation of the rubbery phase (as quantified by its “tan δ” peak) during dynamic mechanical analysis under cyclic deformation.
See Turley and Keskkula (1980), Choi et al. (2000), Coelho et al. (2000), Aiamsen et al. (2003), Qiao-long et al. (2005), Rivera et al. (2006), U.S. Pat. Nos. 5,352,727, 5,436,397 and European Patent No. EP0475461 for some examples of the effects of rubber modification on the mechanical properties. Toyoshima et al. (1997) provide an example of the optimization of the balance between the impact strength and gloss by means of the choice of impact modifier. U.S. Pat. No. 5,475,053 teaches impact-resistant thermoplastic (including HIPS) molding compositions having a matte surface. Cho et al. (1997) show that the environmental stress cracking resistance in the simultaneous presence of a hostile chemical environment along with a mechanical load can also be improved by rubber modification.
4. Rubbers Used as Impact Modifiers for Styrenic Polymers
Many types of rubbers can be and have been used as tougheners for styrenic polymers, with varying levels of effectiveness in improving the properties, practicality of manufacturing, and economic viability in terms of the balance between improved properties and increased cost.
Polybutadiene (dissolved in the reactive monomer mixture after being placed there in a solid particulate form) is the rubbery phase that is most often incorporated as a toughener into polystyrene in order to manufacture HIPS.
Hydroxyl-terminated polybutadiene liquid rubbers have also been used. Their liquid state allows their easy incorporation (with grafting) into polystyrene, with a controlled particle size, during polymerization. See Coelho et al. (2000) for this approach.
The effects of using polybutadienes of different chain microstructure (different cis-1,4, trans-1,4, and vinyl-1,2 isomer contents) have also been investigated. For example, it has been shown by Rivera et al. (2006) that some polybutadiene microstructures provide noticeably more favorable balances between impact modification and the other mechanical properties.
Natural rubber, poly(alkyl acrylate) rubbers, partially or completely hydrogenated diene rubbers, and olefinic rubbers, are some other examples of rubbers that have been used as impact modifiers. For example, see Aiamsen et al. (2003) for the use of radiation-crosslinked natural rubber, Qiao-long et al. (2005) for the use of nanosilica-containing poly(butadiene styrene) rubber, European Patent No. EP0475461 for the use of a selectively partially hydrogenated polybutadiene, and U.S. Pat. No. 5,847,054 for the use of olefinic rubbers.
It is also worth noting that some of the terms that may be used for the different types of rubbers overlap. Sometimes, they may refer to the same type of molecular structure obtained in different ways. For example, a completely hydrogenated polybutadiene or a completely hydrogenated polyisoprene has the same general type of molecular structure as a polyolefin. Both are fully saturated aliphatic hydrocarbons. The difference is that one is obtained by polymerizing butadiene or isoprene and hydrogenating the resulting polymer; while the other is obtained directly by reacting olefinic monomers such as ethylene, propylene and/or 1-butene.
Various block copolymers (such as styrene-butadiene or styrene-isoprene diblock and styrene-butadiene-styrene or styrene-isoprene-styrene triblock copolymers and their partially hydrogenated versions) have been used either as impact modifiers on their own or as compatibilizers between polystyrene and an impact modifier such as polybutadiene. For some examples, see Conway et al. (1997), Cho et al. (1997), Toyoshima et al. (1997) and Aiamsen et al. (2003). A method using block copolymers as impact modifiers incorporated into crosslinked styrenic polymer particles is taught in U.S. Pat. No. 5,847,054. Impact-modified syndiotactic polystyrene compositions containing block copolymers are taught by U.S. Pat. Nos. 5,352,727 and 5,436,397; while U.S. Pat. Nos. 5,380,798, 5,475,053 and World Patent No. WO9607675 teach some other examples of the use of block copolymers as impact modifiers for styrenic polymers.
The rubbers used in the impact modification of styrenic polymers include both thermoset elastomers and thermoplastic elastomers. Thermoset elastomers (usually more simply referred to as “rubbers”), such as crosslinked polybutadiene and crosslinked polyisoprene, have a covalently crosslinked three-dimensional network structure, but possess a low glass transition temperature [below “room temperature” (25° C.)]. Thermoplastic elastomers, such as styrene-butadiene diblock copolymers and styrene-butadiene-styrene triblock copolymers, contain soft domains that are “physically crosslinked” by hard domains, while they lack a covalently crosslinked three-dimensional network structure. Both thermoset elastomers and thermoplastic elastomers are used simultaneously in some formulations for the impact modification of styrenic polymers. An example is the use of a crosslinked polybutadiene as the main component of the impact modifier, along with some thermoplastic styrene-butadiene diblock copolymer that serves mainly as a compatibilizer between the styrenic phase domains and the polybutadiene phase domains.
If there is a significant reactivity difference between the styrenic monomer(s) and the other type(s) of monomer(s) present in a reactive mixture, then there is a tendency towards the formation of a heterogeneous morphology [with domains rich in the styrenic polymer and domains rich in the product(s) of the polymerization of the other type(s) of monomer(s)] even when the reaction is started with a mixture of monomers rather than incorporating the non-styrenic component in an oligomeric or polymeric form. See, for example, Lu and Larock (2006). This article also illustrates the utilization of a renewable resource (corn oil) as a source of monomers for use in the preparation of polymer composites. The growing use of renewable resources as feedstocks will be discussed further in the next paragraph.
A background paper on biopolymers, published by the U.S. Congress, Office of Technology Assessment (September 1993), suggested that the use of biologically derived polymers could emerge as an important component of a new paradigm of sustainable economic systems that rely on renewable sources of energy and materials. This concept has, indeed, gained increasing acceptance in the years that followed the publication of the background paper. The utilization of monomers obtained from biological starting materials (such as amino acids, nucleotides, sugars, phenols, natural fats, oils, and fatty acids) in the chemical synthesis of polymers is an important component of this paradigm of sustainable development. This is an area of intense research and development activity because of the global drive to reduce the dependence of the world economy on petrochemical feedstocks. Such renewable feedstocks can be obtained from a wide variety of microorganism-based, plant-based, or animal-based resources. The utilization of monomers, oligomers and polymers obtained from renewable resources as components of polymer composites is, therefore, anticipated to continue to increase in the future. Among renewable feedstocks for the synthesis of polymeric products, natural fats and oils extracted from some common types of plants [such as soybean, sunflower, canola, castor, olive, peanut, cashew nut, pumpkin seed, rapeseed, corn, rice, sesame, cottonseed, palm, coconut, safflower, linseed (also known as flaxseed), hemp, castor bean, tall oil, and similar natural fats and oils; and especially soybean, sunflower, canola and linseed oils] appear to be very promising as potential sources of inexpensive monomers. Some animal-based natural fats and oils, such as fish oil, lard, neatsfoot oil and tallow oil, may also hold promise as potential sources of inexpensive monomers. U.S. Patent Application No. 20050154221 teaches integrated chemical processes for the industrial utilization of seed oil feedstock compositions. An article by Pillai (2000) discusses the wealth of high value polymers that can be produced by using constituents extracted from cashew nut shell liquid. Belcher et al. (2002) show that the blending of functionalized soybean oil with petrochemical-based resins can increase the toughness of a petroleum-based thermoset resin without compromising stiffness, while also improving its environmental friendliness.
5. Methods for Manufacturing Impact-Modified Styrenic Polymers
In toughening styrenic polymers by incorporating polybutadiene, the most common preparation method is bulk-suspension copolymerization. In applying this method, a prepolymer is first prepared by using bulk polymerization. The preparation of HIPS in the form of beads (or pellets) is then completed via suspension polymerization. It is, however, also possible to use bulk polymerization by itself, or (as taught, for example, in U.S. Pat. No. 4,730,027) suspension polymerization by itself from the beginning to the end to prepare HIPS.
Batch polymerization is most commonly used, but methods (such as the one taught in U.S. Patent Application No. 20030083450) are available for continuous polymerization.
When suspension polymerization is used (either by itself or after bulk polymerization), substantially spherical polymer beads of a wide variety of desired diameters can be produced by varying the process parameters (and especially the stirring rate).
The details of the formulation and processing conditions play crucial roles in determining the extent of grafting, as well as the size distribution and morphology of the rubbery domains. For example, the stirring (shear) rate is a process variable that has been used to control the dispersed rubbery domain size. Faster stirring normally results in smaller rubbery domain sizes. The effects of processing conditions on the morphologies of heterophasic polymeric materials (including toughened polymers and toughened polymeric composites) have been discussed by Bicerano (2002) in terms of the interplay between thermodynamic and kinetic factors.
One usually obtains thermoplastic pellets of HIPS with most of the approaches that are practiced since there is no crosslinker in the typical HIPS formulation. These thermoplastic HIPS pellets can then be melted for processing via techniques such as molding or extrusion into fabricated articles of desired shapes and sizes.
6. Optimum Incorporation of Impact Modifiers in Styrenic Polymers
There is an optimum incorporated rubber particle size. This size depends upon various factors. It typically ranges from 1 to 3 microns in diameter. See Toyoshima et al. (1997), Bicerano (2002) and Aiamsen et al. (2003) for discussions of the effects of rubber particle size.
The use of 5% to 15% by weight of polybutadiene (with 7% by weight being viewed as an optimum value by some experts) is the most common approach. However, the rubbery phase volume fraction in HIPS is typically much higher than the weight fraction of the rubber since the rubbery phase domains also normally contain a lot of occluded polystyrene. For further discussions of the optimum rubber weight fraction and/or rubbery phase volume fraction, see Turley and Keskkula (1980), Cho et al. (1997), Choi et al. (2000) and Aiamsen et al. (2003).
In general, much better impact modification is obtained if the rubbery material becomes covalently bonded to (grafted onto) the styrenic polymer chains during preparation rather than just being physically blended into the polymer. See Cho et al. (1997), Choi et al. (2000), Qiao-long et al. (2005) and Rivera et al. (2006) for examples of this effect. This is why suspension polymerization techniques, which lead to the grafting of an impact modifier containing one or more reactive functionalities onto the matrix polymer, are normally preferred to approaches such as the melt blending of a rubber into polystyrene.
A reactive impact modifier can be incorporated into the formulation as a monomer, as an oligomer, or as a polymer. The use of a reactive oligomer or polymer as the impact modifier can cause the mixing of the impact modifier into the formulation to become more difficult than the use of a monomeric impact modifier of similar chemical structure, especially if the reactive oligomer or polymer is a solid. However, once a reactive oligomer or polymer impact modifier is mixed adequately with the other components of the formulation, this approach may offer the advantage of the more facile attainment of a heterophasic morphology where the impact modifier is present in phase-separated domains that provide an optimum toughening effect with the least possible reductions of other important properties such as stiffness (modulus) and strength.